1. Field of the Invention
This invention relates to a semiconductor magneto-optical material whose magneto-optical effect can be utilized to enable optical communication, signal processing and data recording by laser beam or the like.
2. Description of the Prior Art
Recent advances in next-generation optical communication technology and next-generation large-capacity data storage technology, together with ever expanding networks offering commercial connection services and the like via the Internet, are combining to set the stage for a full-blown multimedia age that is rapidly approaching.
The core technologies required for next-generation optical communications are:
1. Ultrahigh speed/ultra-long distance transmission technology PA1 2. Optical coherent communication technology PA1 3. Optical signal processing technology PA1 4. Optical component and integrated circuit technology.
Of recent technological breakthroughs, one of the most significant in connection with items 1-4 is the erbium-doped fiber amplifier (EDFA). Owing to its larger repercussion effect, the EDFA has dramatically increased transmission distance.
A laser diode (LD) module is used as the excitation light source and the signal light source of the EDFA. When a laser module is used, however, light reflected by, for example, the end face of the optical fiber connected with the laser module and the connection points between optical fibers reenters the LD. The operation characteristics are therefore markedly degraded by the occurrence of retrogressive light noise, output fluctuation, and other factors. The practice is therefore to block the retrogressive light reflected toward the LD by use of an optical isolator so as to overcome the operational instability of the LD module owing to reflected retrogressive light.
An optical isolator is an optical component (optical nonreciprocal circuit) using a magneto-optical material exhibiting magneto-optical Faraday effect. In the field of optical communication, optical isolators have been developed for various wavelengths, including the 0.8 .mu.m band (the wavelength of the most inexpensive GaAs semiconductor laser), the 1.3 .mu.m-1.5 .mu.m band (the band of lowest optical fiber transmission loss) and the 0.98 .mu.m band (used for high-efficiency EDFA excitation).
Except in the 0.98 .mu.m band, the typical magneto-optical material used is bismuth-substituted garnet.
Since the optical absorption of bismuth-substituted garnet is large in the 0.98 .mu.m band, however, another magneto-optical material has been sought. This led to the recent development of a practical bulk isolator using a magnetic semiconductor based on cadmium telluride, a II-VI group semiconductor.
On the other hand, the optical isolator continues to account for a major portion of optical amplifier size and cost. In view of plans to connect individual homes with optical communication networks for introduction of bidirectional interactive services, multimedia communication services and the like, a strong need is felt for a smaller, low-cost optical isolator and for an optical waveguide-type optical isolator in the form of a thin film on the surface of a substrate. It will be immeasurable if this need should be met.
When a magnetic semiconductor based on cadmium telluride is used as the magneto-optical material of the optical isolator, the magneto-optical material must have a thickness of around 1,400 .mu.m in order to secure a 45-degree rotation angle of the polarization surface. This makes it difficult to achieve small size and low cost.
An attempt to fabricate an optical isolator as an optical waveguide-type optical isolator encounters considerable difficulty in realizing the optical waveguide since bismuth-substituted garnet and cadmium telluride are poorly compatible with the GaAs semiconductor of the substrate.
An optical switching element utilizing the magneto-optical effect is also desired, not only for use in optical isolators but also for realizing optical integrated circuits, optical computers and the like. A magneto-optical material that can overcome the foregoing problems is therefore also sought for this purpose.
This invention was accomplished in light of the foregoing circumstances and has as its object to provide a semiconductor magneto-optical material that exhibits pronounced magneto-optical effect in a desired wavelength region and can be formed as a thin film.